- #1
piepowah
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Homework Statement
Let W be the region bounded by y + z = 2, 2x = y, x = 0, and z = 0. Express and evaluate the triple integral of f (x, y, z) = z by projecting W onto the: (a) xy-plane (b) yz-plane (c) xz-plane.
Homework Equations
The function f (x, y, z) = z and the boundary W: {y + z = 2, 2x = y, x = 0, and z = 0}
The Attempt at a Solution
∫∫∫ z dzdydx; {w: 0 < x < 1, 0 < y < 2x, 0 < z < 2 - y}
w
= ∫∫1/2z^2 dydx z from 0 to 2 - y
= 1/2∫∫ (4 - 4y + y^2)dydx
= ∫(2y - y^2 +1/6y^3) dy y from 0 to 2x
= 2x^2 - 4/3x^3 + 1/3x^4 x from 0 to 1
= 2 - 4/3 + 1/3 = 1
b. yz-plane
∫∫∫ z dxdzdy; {w: 0 < y < 2, 0 < z < 2 - y, 0 < x < y/2}
w
= ∫∫xz/2 dzdy, x from 0 to y/2
= 1/4∫ yz^2 dy, z from 0 to 2 - y
= 1/2y^2 - 1/3y^3 + 1/16y^4, y from 0 to 2
= 2 - 8/3 + 1 = 1/3
c. xz-plane
∫∫∫z dydzdx; {w: 0 < x < 1, 0 < z < 2 - 2x, 2x < y < 2 - z}
w
= ∫∫yz dzdx, y from 2x to 2 - z
= ∫∫(2z - z^2 - 2xz) dzdx
= ∫(z^2 - 1/3z^3 - xy^2) dx, z from 0 to 2 - 2x
= ∫(4 - 8x + 4x^2 - 1/3 (2 - 2x)^3 - x(4 - 8x +4x^2)dx
= (4x - 4x^2 + 4/3x^3 - 2x^2 + 8/3x^3 - x^4), x from 0 to 1
= 4/3 - 2 + 8 /3 - 1 + 16/24
= 5/3
I think that the result is a four-dimensional value, finding the volume over the function
f ( x, y, z) = z. However, the results should come out to be the same value, but in these three cases I don't. I visualize the picture to be a pyramid with a triangular base. I have a feeling I messed up the limits of integration and have tried rewriting the limits of integration many times and have come up with values that don't match up. Can somebody tell me the correct limits of integration for the triple integral and why the ones that I have are incorrect?
Thanks.
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